The structure and operation of wireless communication systems are generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks. Equipment deployed in these communication systems is typically built to support standardized operating standards that prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, MAC layer operations, link layer operations, etc.
In a cellular system, a regulatory body typically licenses a frequency spectrum for a corresponding service area utilized by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies, i.e., channels, within the frequency spectrum that support the subscriber units within the service area. Typically, these channels are equally spaced across the licensed spectrum. The separation between adjacent carriers may be defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference. In most cases, severe limitations are placed upon the amount of co-channel and adjacent channel interference that may be caused by transmissions on a particular channel.
In a cellular system, a plurality of base stations may be distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., Global System for Mobile Communications (GSM) cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies, and reverse link communications (from subscriber units to the base station) on a second set of carrier frequencies. The first set and second set of carrier frequencies supported by the base station are a subset of all of the carriers within the licensed frequency spectrum. In most cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized and system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results.
Traditional wireless mobile networks include mobile switching centers (MSCs), base station controllers (BSCs) and base stations that jointly operate to communicate with mobile stations over a wireless communication link. Examples of common networks include the GSM networks, North American Time Division Multiple Access (TDMA) networks and Code Division Multiple Access (CDMA) networks. Extensive infrastructures (e.g., ANSI-41 or MAP-based networks) exist in the cellular wireless networks for tracking mobility, distributing subscriber profiles, and authenticating physical devices. In wireless mobile networks providing a facility to determine a mobile device's geographic position, a network component commonly referred to as a mobile location center (MLC) or geolocation subsystem (GLS) performs a location calculation.
To establish a wireless communication link in traditional wireless voice networks, an MSC communicates with a BSC to prompt the base station to generate paging signals to a specified mobile device within a defined service area, i.e., a cell or a sector. The mobile device, upon receiving the page request, provides a response indicating it is present and available to accept an incoming call. Thereafter, the base station, upon receiving a response from the mobile device, communicates with the MSC to advise the MSC of the same. The call may then routed through the base station to the mobile device as the call setup is completed and the communication link is created. Alternatively, to establish a call, a mobile device generates call setup signals that are processed by various network elements in a synchronized manner to authenticate the user as a part of placing the call. The authentication process may include, for example, communicating with a Home Location Register (HLR) to obtain user and terminal profile information. The HLR is a central database that stores the permanent parameters of the user including additional services, the encryption keys for digital signal transmission, and the address of a Visitor Location Register (VLR) database. The VLR database contains information associated with the mobile device's current location including the serving base station.
In 1996, the Federal Communications Commission (FCC) issued a report and order requiring all wireless carriers and mobile phone manufacturers to provide a capability for automatically identifying to emergency dispatchers the location from which a wireless call was made. Implementation is divided into two phases. Phase I requires wireless service providers and mobile phone manufacturers to report the telephone number of the mobile device making the call as well as the base station servicing the mobile device which provided a general area from which the call was made. This information can be obtained from the network elements. Phase II of the FCC's Enhanced 911 (E-911) mandate states that by Oct. 1, 2002, wireless service providers must be able to pinpoint, by latitude and longitude, the location of a subscriber who calls emergency 911 from a mobile device. Wireless service providers were given the option of providing a network-based solution or a handset based solution.
One known method for locating a mobile device is triangulation. Signal power level or signal timing measurements between the mobile device and three or more base stations may be used to triangulate. Signal power levels or signal timing measurements may be used to estimate the distance between each base station and the mobile device. The distances are plotted to determine a point of intersection, and the point of intersection is the approximate transmitter location. For calculations using only signal power measurements, this method works only when the signal strength is relatively strong and not greatly affected by radio frequency (RF) fading, such as multi-path interference. RF fading occurs when radiated signals encounter various obstacles that reflect and diffract the signal causing the received signal power level at the base station and mobile device to vary. The requirement for a minimum of three base stations and the effect of RF fading limits the usefulness of triangulation.
Location techniques relying on measurements of timing differences, such as time difference of arrival (TDOA) or enhanced observed time difference (E-OTD), require signal timing measurements between the mobile device and three or more separate base stations. If the wireless network's base stations are not synchronized, then additional equipment is required at each base station to measure the timing difference between base stations in the network. If the wireless network is not capable of collecting signal timing measurements between three or more base stations and the mobile device, modification of the base station, and optionally the handset, are required. The modification of base stations and/or handsets implies significant additional cost to wireless network operators.
The Global Positioning System (GPS) provides a means to fix a position of a transmitter using a system of orbiting satellites with orbital planes that guarantee at least four satellites are visible at all times. This system provides location accuracy to within one meter for military systems possessing a Selective Availability (SA) algorithm to filter out the intentional noise added to the signal. GPS systems without SA are limited to an accuracy of approximately 100 meters. Widespread use of the GPS and the decision to discontinue the LORAN-C navigation system convinced the Department of Defense to drop SA thereby allowing commercial GPS receivers to dramatically increase accuracy. The FCC recognized that GPS receivers could be incorporated into mobile phones when it made minor adjustments to the Phase II schedule. Using GPS to report location, however, requires a mobile device user to upgrade existing hardware or to purchase new hardware and/or software. One of the largest obstacles to implementing such a system is large costs associated with deploying and maintaining necessary hardware and software needed to bring this technology into use.
Thus, there exists a need in the art for a method and system to calculate a mobile device's location that avoids the limitations of the prior art while limiting the impact to users and to network operators. The geographic location of active wireless mobile devices in a network is of great interest for fulfillment of the FCC E-911 regulations, and necessary for offering location based services. Embodiments of the present subject matter address the cost and complexity issue by offering a novel method of locating wireless users using measurements from only a single wireless sensor device. Thus, embodiments of the present subject matter offer cost and complexity advantages over conventional network-overlay location methods such as TDOA, E-OTD and Angle of Arrival (AOA) that rely on signal reception and measurement from multiple sensors to locate mobile devices.
Accordingly, it is an object of the present subject matter to obviate many of the deficiencies in the prior art and to provide a novel method of locating a position of a mobile device using a sensor spaced apart a known distance from a base station in communication with the mobile device where the mobile device provides a signal transmission. The method further comprises calculating and choosing a set of coordinates for the mobile device relative to the sensor as a function of the known distance from the sensor to the base station, a range between the mobile device and the base station, an estimate of base station transmission timing relative to a time source known at the sensor, and a time of arrival of the signal transmission at the sensor.
It is also an object of the present subject matter to provide a novel method for determining a position of a wireless mobile device comprising the steps of providing one base station in communication with the mobile device, providing a sensor spaced apart a known distance from the base station, and determining the range between the mobile device and the base station. The method further comprises the steps of measuring the time of arrival at the sensor of a signal transmitted from the mobile device, calculating a set of coordinates for the mobile device relative to the sensor as a function of the known distance from the sensor to the base station, the range, an estimate of base station transmission timing relative to a time source known at the sensor, and the time of arrival, and choosing a one of the set of coordinates to thereby determine a position of the mobile device relative to the sensor.
It is a further object of the present subject matter to provide a novel system for locating a position of a mobile device. The system comprises a mobile location center in communication with a network having at least one sensor spaced apart a known distance from a base station in communication with the mobile device, the mobile device providing a signal transmission. The mobile location center calculates and chooses a set of coordinates for the mobile device relative to the sensor as a function of the known distance from the sensor to the base station, a range between the mobile device and the base station, an estimate of base station transmission timing relative to a time source known at the sensor, and a time of arrival of the signal transmission at the sensor.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.